1. Field of the Invention
This invention relates to light beam deflecting apparatus, and more particularly to such apparatus in which a light beam is deflected or swept by means of a hologram.
2. Description of the Prior Art
Light beam scanning technique utilizing the nature of a hologram has many advantages over that employing a rotating polygonal mirror. For example, a hologram scanner is producible by an unsophisticated machining operation, and may considerably be reduced in size because the hologram itself has the focusing function without light condensing lenses. In addition, a number of copies of holograms can be prepared from a single original hologram.
While the hologram scanner can be used in conjunction with a variety of image recording and read-out apparatus such as a laser printer or a facsimile system, it has not been possible with the conventional hologram scanner to accomplish a perfectly straight-line scanning by which a straight scanning line is formed on a flat scanning plane.
In a facsimile system, for instance, both a document and a recording sheet of paper as recording media are usually placed in flat positions. If the system were adapted, to curvilinear scanning, then not only would the overall system be complex in structure, but increased in cost, complicated in handling of the recording media and lowered in operability of the system. Therefore, flat surface scanning should preferably be employed in conjunction with a image recording and read-out system of the type as mentioned above.
A hologram is usually prepared by irradiating a sheet or plate of photosensitive material with a plane or spherical wave beam as a reference beam together with a spherical wave beam emanating from a fixed point as a signal beam to form a pattern of diffractive gratings on the photosensitive sheet or plate. If a reproducing illuminating light emanating from such a fixed point is employed to deflect a scanning beam by means of a rotating hologram, then the focal point or converging point of the reproduced scanner light beam is kept positioned at a predetermined distance from the center of rotation of the hologram since the point from which the spherical wave light emanates is fixed. Therefore, when the scanner beam reproduced from the hologram is deflected or swept by changing the relative position between the hologram and the reproducing illuminating light, for example, by rotating a hollow cylinder of which on the cylindrical surface a hologram is formed, the converging points of the scanner light beam delineate an arc of a circle, failing to accomplish a desired flat surface scanning.
In the case of rotating a disk on which a hologram is formed with a reproducing illuminating light impinging thereupon, the curve delineated by the converging points of the scanner light beam is more complex. In this case, the scanner light beam is kept formed on the lateral surface of a cone frustrum having an apex on the rotational axis of the disk hologram. Therefore, not only does the focal point of the scanner light beam describe an arc, but the scanner beam itself is not formed, in this case, on one flat scanning plane. Therefore, when the scanning beam describing the lateral surface of a cone in this manner is used for scanning a flat image, a reproduced image may be distorted.